New Integral Inequality Approach on Stability Criteria for Delayed Load Frequency Control Systems

In wide-area interconnected power systems, load frequency control (LFC) has a pivotal role in addressing the issue of upholding the variations in system frequency and power exchange between different control areas at desired scheduled values. With the development of smart grid technologies and the emergence of numerous private networks, open communication networks are widely used, which inevitably leads to communication delay. The communication delay is an important factor affecting the stability and performance of LFC system. At present, Lyapunov-Krasovskii (L-K) functional method is one of the main methods to study the stability of LFC system. To this issue, how to construct appropriate L-K functional and estimate the functional derivatives accurately to reduce the conservatism of conclusions is the core problem, and the sustained efforts have been made, but it is far from enough in how to coordinate functional construction with estimating techniques efficiently. In this paper, the stability problem of LFC system with delay influence and load disturbance is further studied, by proposing some new methods, the less conservative stability criteria are obtained, and the influence of controller parameters on the delay margin is analyzed. Firstly, by considering time delay and load disturbance, the LFC system model is established. Secondly, a new L-K functional with augmented vector and multiple integral terms is constructed. The single integral terms and the double integral terms are introduced, which builds more relations among different vectors. Moreover, state vector and its derivative are augmented to deepen the relationships between L-K functional and the system. Besides, integral functionals with double and triple forms are also created to get a better exploitation of delay information, all of which conduce to the stability criteria with less conservatism. Then, in order to cooperate with the constructed functional effectively, two inequalities named delay-dependent-matrix-based and free-matrix-based integral inequality (DDMB and FMB integral inequality) and extended delay-dependent-matrix-based reciprocally convex inequality (EDDMB reciprocally convex inequality) are proposed to estimate the functional derivatives more accurately. Compared with the existing estimation approaches with constant matrices, DDMB and FMB integral inequality employs delay-dependent matrices and utilizes more information of time delay and its derivative, which provides more freedom in reducing the conservatism of the main results. Compared with the existing DDMB reciprocally convex inequality, EDDMB reciprocally convex inequality is more general because the value of delay lower bound is relaxed, which expands its scope of application. In addition, auxiliary-function-based integral inequalities (AFBII) together with relaxed integral inequality are also used, which helps to get less conservative stability conditions. The simulation of typical second-order delay system and single-region delay LFC system are given, and the influence of load disturbance and controller parameters on the upper bound of single-region delayed LFC system under different conditions is analyzed. The following conclusions can be drawn from the simulation analysis: (1) Compared with the research results of some existed literatures, the upper bound of time delay obtained by the proposed method is significantly larger, which indicates that the proposed method can effectively improve the stability margin of time delay and reduce the conservatism of the conclusion. (2) The influence of load disturbance and controller parameters on the upper bound of system delay is obvious, the larger the load disturbance is, the smaller the upper bound of system delay is. When the PI control is applied, the upper bound of time delay decreases with the increase of integral gain, and this trend is more obvious when the proportional gain is smaller. The relation between the upper bound of time delay and the proportional gain is more complicated, the upper bound of time delay increases first and then decreases with the increase of the proportional gain. © 2023 China Machine Press. All rights reserved.